Very Large Scale Integration (VLSI) and Ultra Large Scale Integration (ULSI) devices in semiconductor technology are expected to result in significant performance improvements in systems based on such devices. However, many of the advantages of such devices will be lost without significant improvements in device packaging techniques. A number of new designs, materials and processes capable of providing high density multilayer interconnections with controlled electrical characteristics have been proposed. These include high density printed circuit boards, cofired multilayer ceramics, customized multilayer copper/polyimide interconnections built on cofired ceramic substrates and wafer scale integration using semiconductor integrated circuit (IC) processing. One of the most versatile and cost effective of the interconnection technologies is a sequentially built multilayer structure of metal conductor and dielectric material using a substrate such as silicon, ceramic or molybdenum.
However, one of the problems encountered in the multilayer structure is the ability to obtain continuity of the upper layer metal films and the integrity of the dielectric insulating layer over the step formed by the underlying metallization pattern. The problem is caused by the protrusion of the metallization pattern that results in an increase in the degree of unevenness as the number of metallization layers increases.
Another problem relates to the adhesion between the metal conductor and the dielectric. For instance, the adhesion between a copper conductor and polyimide dielectric is enhanced by the use of a thin layer of chromium, titanium, titanium tungsten alloy or nickel between them. This thin layer of metal also serves as a corrosion barrier for the copper conductor. The two techniques previously known in the art, the subtractive and the additive process, were unable to provide a suitable layer between the conductor and the dielectric. The subtractive process did not provide a completely sealed and corrosion resistant copper conductor; and although the additive process formed a completely sealed and corrosion resistant copper conductor, extra processing steps were required, which resulted in extra expense.